QTat Alluvial terrace deposit (Pleistocene and Pliocene?)—Light-brown, yellowish- brown, brown, and light-gray gravel, sand, silt, and clay at elevations higher than Qat. Alluvium of several terraces of different ages and elevations included in this unit. Crudely to well stratified, and poorly to moderately well sorted. In places deposits have been cemented to conglomerate by calcium carbonate or iron oxide. Nearly all clasts are well rounded and composed dominantly of quartzite, chert, volcanic rocks, ironstone concretions, sandstone, and siltstone with minor amounts of shale, agate, silicified wood, jasper, chalcedony, and clinker. Clasts are as much as 15 inches in diameter, but most less than 2 inches. Thickness about 30 ft.
QTcl Clinker (Holocene, Pleistocene, and Pliocene?)—Red, pink, orange, black, and yellow, resistant metamorphosed sandstone, siltstone, and shale of the Fort Union Formation. Bedrock was baked by natural burning of adjacent coal bed. Locally, baked rock was melted and fused to form buchite, a black, glassy, vesicular or scoriaceous rock. Unaltered glacial till lies directly on thick clinker in parts of the map area indicating that some of the beds must have burned before the advance of the ice sheet into this area (Prichard and Landis, 1975). Thickness as much as 60 ft.
Fort Union Formation (Paleocene) Sentinel Butte Member—Dark-gray shale locally underlain by orangish brown or brown, iron oxide-stained, cross-bedded channel sandstone with medium- to coarse-grained, angular to subangular, poorly sorted clasts. Channel bases display scour features including abundant rip-up clasts. Gray or grayish-brown, poorly resistant mudstone about 20 ft thick underlies the sandstone locally. Prominent coal beds are present near the base of the member in most areas. Upper part of member removed by erosion. Thickness of about 300 ft exposed in map area.
Tongue River Member—Yellow, orange, or tan, fine- to medium- grained sandstone and thinner interbeds of yellowish-brown, orange, or tan siltstone and light-colored mudstone and clay. Clay dominantly nonswelling. Contains several prominent lignite beds (Mathews, 1989a, b, c, and d). Member generally poorly cemented and weathers to badland topography, but some relatively resistant sandstone beds form caprocks. Locally contains silicified wood, freshwater mollusks, and plant molds. Thickness 850 ft.
Lebo—Medium- to dark-gray, grayish-brown and olive-gray sandstone, siltstone, and mudstone that is typically smectitic or carbonaceous, interbedded with gray to dark-gray, silty shale, thin yellowish-gray siltstone and sandstone, and thin, lenticular lignite beds. Contains small-scale, light-gray, fine- to medium-grained, cross-bedded channel sandstones. Clay typically exhibits characteristic “popcorn” weathering. Thickness about 200 ft.